Best practice for database cross-value storing

Question

I have one table in my db that looks like this:

|    userid    |    username    |    dateloggedin    |    etc
|    1         |    Joebloggs   |    27-03-16        |    etc
|    2         |    Nathanbloggs|    27-03-16        |    etc

and a logging table that looks like this:

|    eventtype |    userid      |    eventdate       |    etc
|    loggedin  |    1           |    27-03-16        |    etc
|    loggedout |    1           |    27-03-16        |    etc

When I display the entries from the eventlog I want to also display the username, not just the userid.

What's the best practice here, should I store the username in both the eventlog table and the users table so I can access all data in a single query? Or should I use the userid to cross reference the other table and get the username from there, only storing it once?

Given that I am planning for volumes I'm not sure which route is best - more processing time from switching between queries and arrays, or more storage space used by duplicating data.

Answer 1

http://www.tutorialspoint.com/sql/sql-using-joins.htm

This link explains the sql JOINS syntax quite well, thanks!